This invention relates to filters cleanable by washing or vacuuming or the like for inlet air heating and air conditioning systems used in residential or commercial buildings and, more particularly, this invention relates to a washable electrostatic filter for such systems having an extended service life and improved efficiency.
Inlet air filters for heating and air conditioning systems have been in use for decades. The inlet air or primary filter was originally intended to protect the heating coils and mechanical devices such as fans from damage by airborne particles. As the harmful effect of inhaled particles on human health became known, inlet air filters were designed to remove this particulate matter from the air.
The most popular prior art configuration is a thin, rectangular, disposable filter. The filter contains fiberglass, animal hair, fibrous foam or polymeric media or aluminum mesh encased in a cardboard or plastic frame. Prior art filters comprised polyester panels, urethane foam or latex coated animal hair. Recently, prior art filters containing static or passive electrostatic media have become available. Most residential resistance specifications require air filters to have an initial pressure drop of no more than 0.22 inches of water for an airflow rate of 300 feet per minute. The dust spot efficiency for typical prior art air filters tested using ASHRAE 52.1-1992 is 20% or less. The ASHRAE efficiencies of four types of commercially available filters follow:
When these filters are first placed in use across an air stream, they have a very low filtering efficiency. Typically, the exhaust or dust spot efficiency is about 8%. However, as dust particles are collected, the dust collection efficiency can increase to approximately 20%. At this point, the filter is ready to be replaced. Prior art filters are usually difficult to clean since the dust particles become embedded in the media.
Another current concern is the recognition that particles below 10 microns are not filtered by the cilia hairs in the nose and are therefore inhaled into a human lung. Fibrous particles such as asbestos and fiberglass are known to cause respiratory diseases. Most current inlet air filters for heating and air conditioning systems are not very efficient in capturing these small particles. Filters containing layers of electrostatic media perform better within this range of particles but these filters also become clogged. Since they cannot be efficiently cleaned they must be replaced.
U.S. Pat. No. 6,056,860 disclosed that pleating a washable electrostatic filter resulted in increased efficiency. Capture of particles was enhanced by the angle of approach to the pleated media and by the use of passive electrostatic netting in the multi-layer media. An electrostatic field is only present when air flows past the passive netting and induces charge on the surface of the netting.
The next generation of electrostatic type filter requires an efficiency boost, without increasing the amount of pressure it takes to push the air through the filter medium. Today, passive electrostatic filter medias work mainly on the principal of friction and inherent static electricity in the polymer to obtain their efficiencies. A company that manufactures static control equipment has stated that, contrary to popular belief, static electrification is never caused by just an air flow hitting a solid surface. In most cases, the filters contain layers of a polypropylene honeycombed netting, typically woven, formed from non-woven extruded, monofilament approximately 0.01 diameter and a central layer of a urethane foam or a high loft polyester which also contains some inherent static charges. In the more efficient static charged fiber filters, each fiber has both positive and negative charges. These fibers are 3 to 30 denier in size, or approximately 0.00049 to 0.00268 in. diameter. And usually these types of filters are expensive and are of a disposable type lasting about three months. These filters are known as an xe2x80x9cElectret typexe2x80x9d having efficiencies of approximately 34% dust spot.
The next generation of washable filters requires an increase in efficiency without increasing the pressure drop across the filter.
The efficiency of washable filters is further improved in accordance with this invention by adding active electrostatic materials to the polymeric netting, and/or central filter material and/or to the adhesive binder resins used to bond the fibers of the central filter layer of the filter. Preferred active electrostatic materials are charge control agents. The filters of the invention can have a flat or pleated configuration. Pleated filters result not only in increased efficiency but also lower initial resistance to flow. The life of the filter between cleanings is substantially increased and cleaning the filter is much easier. Another aspect of the invention is method and apparatus for forming and charging multiplayer washable filter media containing charge control agents.
In the filter of the invention, charge is not only induced on the surface of the netting or central layer, but is present throughout the bulk of the polymer fibers in the netting or the polymer fibers or foam in the central layer. The polymers are preferably non-conductive and contain from 0.5% to 15% preferably 1 to 10 percent by weight of the charge control agents. The polymers must have high melting and decomposition temperatures to survive the compounding, charging, pleating and other steps of manufacturing the filters. Suitable polymers are polyalkylene polymers having 2-6 carbon atoms such as polyethylene and polypropylene, linear polyamides such as Nylon(copyright), polycarbonates, and polyurethanes. Polar polymers containing negative charge agents such as acrylic polymers, polymethacrylic acid or alkyl esters thereof, acrylic copolymers such as styrene-methylmethacrylate may be utilized.
Examples of charge control agents are compounds of organic or organometallic charge control compounds, inclusions compounds of cyclic polysaccharides, monoazo metal compounds, alkyl acrylate monomers, alkyl methacrylate monomers, calixarene compounds metallized with an alkali metal or an alkaline earth metal, rhodamine or xanthene-type dyes, polytetrafluoroethylene, alkylene, arylene, arylenealkylene, alkylenediarylene oxydialkylene or oxydiarylene polyacrylic and polymethacrylic acid compounds, organic titanates, quaternary phosphonium trihalozincate salts, organic silicone complex compounds, dicarboxylic acid compounds, cyclic polyethers or non-cyclic polyethers, cyclodextrin, complex salt compounds of amine derivatives, ditertbutylsalicicyclic acid, potassium tetaphenylborates, potassium bis borates, sulfonamides and metal salts thereof, alumina particles treated with silane coupling agents selected from the group consisting of dimethyl silicone compounds, azo dye, phthalic ester, quaternary ammonium salt, carbazole, diammonium and triammonium, hydrophobic silica and iron oxide, phenyl, substituted phenyl, naphthyl, substituted naphthyl, thienyl, alkenyl and alkylammonium complex salt compounds, sodium dioctylsulfosuccinate, sodium benzoate, zinc complex compounds, mica, monoalkyl and dialkyl tin oxides and urethane compounds, metal complex of salicyclic acid compound, oxazolidinones, piperazines, perfluroinated alkanes, fatty acid amides, oleophobic fluorochemical surfactants, Lecigran MT, nigrosine, fumed silica, carbon black, para-trifluoromethyl benzoic acid and ortho-fluoro benzoic acid, poly(styrene-co-vinylpyridinium toluene sulfonate), methyl or butyltriphenyl phosphonium-p-toluene sulfonate, complex aromatic amines, triphenylamine dyes and azine dyes, alkyldimethylbenzylammonium salts.
The charge control agents (xe2x80x9cCCAxe2x80x9ds) operate by tribolectric charging of particles. In general, optimum concentrations of particles are 0.5-65% by weight and effects are superior when CCAs are uniformly dispersed in the adhesive.
A. A lecithin derivativexe2x80x94LECIGRAN MT results in a (+) charge if the contact is a conductor and a (xe2x88x92) charge if the contact is resistive. A recommended polymer is a styrene-methylmethacrylate copolymer. Since this material would be close to the middle of the triboelectric series, it appears that the lecithin merely enhances the ability of the material to either donate or accept electrons depending on the resistivity of the other half of the triboelectric couple. Lecithin and other derivatives are commonly available chemicals.
B. Nigrosine and its derivatives are unusual in that they tend to result in negative polarity charging at low concentrations. They also are readily available commercially.
C. Fumed silica can be hydrophilic or hydrophobic. In the former state it imparts high negative charges to copolymers such as styrene-butylmethacrylate. Even higher negative charges would be expected in polyethylene and polypropylene because of their more negative position in the triboelectric series. Moderate relative humidity variations should not affect the electrostatic charge significantly. Cabot Corporation is one of the principal manufacturers of fumed silica and other silicas.
D. There is an increasingly popular theory that triboelectric charging is due to the transfer of ions from one surface to another. Materials like polyacrylic and polymethacrylic acids, poly (styrene-co-vinylpyridinium toluene sulfonate), methyl or butyl triphenyl phosphonium-t-toluene sulfonate (Eastman Chemical), complex aromatic amines, some quaternary ammonium salts, triphenylamine dyes and azine dyes are all charge control agents. It has been shown that when the bulkiest part of the CCA molecule or polymer is a cation, the resulting triboelectric charge is (+). Conversely, when the bulkiest part of the CCA material is an anion the triboelectric charge is (xe2x88x92). The small counterion transfers to the other triboelectric material producing the opposite polarity charge. Therefore acids, complex amines, several dyes, and quaternary ammonium salts charge positively; while sulfonates, whether polymeic or not, and similar compounds charge negatively. Hodogaya Chemical Company, LTD produced materials used specifically to control charge magnitude and polarity. Several other chemical companies make materials that can serve the same purpose.
E. Slightly conductive particles at low enough concentrations such that there is little or no particle to particle contact can acquire high triboelectric charges. Carbon blacks are a good example. One variety is an oxidized, acidic carbon black, which tends to acquire a negative charge. Unoxidized carbon black, on the other hand, when dispersed in the same polymer, acquires a positive charge. Thus, it may be possible to use only one polymer micro-fiber and control charge polarity with different CCAs. Such an approach could lead to substantial material production and processing cost savings.
F. Hodogaya Chemical Company, LTD appears to have a rather substantial research program on innocuous CCA materials. This company reports that benzoic acid derivatives are effective charge control agents. Para-trifluoro methyl benzoic acid produces very high (+) charges but ortho-fluoro benzoic acid yields the highest (xe2x88x92) charges.
The filter of the invention is optimally angled to capture particles. It can be cleaned by shaking, vacuuming and/or by washing. The media has low-pressure drop and high efficiency for the range of particles experienced with the inlet air. Because it can be cleaned and reused, the filter of the invention can be considered a active filter.
Other advantages of the media of the invention is the use of thinner central layers 0.1 to 0.5 inches thick with higher surface area and pleating density suitably up to 5.0 square foot area and up to 20 pleats per foot. It is also preferred that the netting contain the opposite charge to the adjacent central layer.
These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.